9:15 AM - 9:30 AM
[HGM03-02] Adjustment timescales of channel width and hillslope angle to accelerated uplift: Insights into a basin-scale steady state
Keywords:Bedrock river, erosion, stream power model, cosmogenic nuclide
River morphology is dictated by various factors, such as substrate properties and climate, and changes in the boundary conditions trigger river adjustment to the new boundary conditions.
A sustained increase in rock uplift rate drives changes in river morphology. Studies of transient landscapes are crucial to understand the style and timescale of river adjustment to accelerated uplift. Channel slope, width, and hillslope angle play important roles in controlling river erosion rate, and their relationships with active tectonics have been examined in detail. While adjustment time of channel slope can be estimated empirically and theoretically based on knickpoint travel time, those of channel width and hillslope angle are poorly understood because they are difficult to constrain in an actual landscape. This study presents a method to estimate adjustment timescales of width and hillslope angle and apply the method to rivers around the Yunodake fault, an active normal fault in Fukushima, northeastern Japan.
We focused on channel slope (ksn: Normalized channel steepness), channel width, hillslope angle and examined their variation along the trunk streams. Rivers flowing across the fault contained slope-break knickpoints. The lower channel reaches had larger ksn than reaches upstream of the knickpoints, suggesting these rivers are now adjusting to the accelerated relative uplift rates of the Yunodake fault. We measured bankfull width using a laser range finder. Although channel width increases with drainage area at a steady state, the observed channel width for three trunk streams did not follow such scaling relations. As for hillslope angle, we mapped hillslopes adjacent to the trunk streams and examined its along-trunk variation of hillslope angles using 5m-DEM provided from the Geospatial Information Authority of Japan. The result showed hillslopes downstream were steeper than those upstream, and there were transitional zones at the middle of the rivers where hillslope angles gradually increased toward the downstream side. We analyzed the 10Be concentrations of detrital sand taken from the trunk streams to determine basin-averaged denudation rates and then calculated knickpoint travel speed using the resulting denudation rates. Based on the observations of channel and hillslope morphology and the knickpoint travel speed, we will discuss the adjustment timescales of channel width and hillslope angles and their significance in terms of a basin-scale steady state.
A sustained increase in rock uplift rate drives changes in river morphology. Studies of transient landscapes are crucial to understand the style and timescale of river adjustment to accelerated uplift. Channel slope, width, and hillslope angle play important roles in controlling river erosion rate, and their relationships with active tectonics have been examined in detail. While adjustment time of channel slope can be estimated empirically and theoretically based on knickpoint travel time, those of channel width and hillslope angle are poorly understood because they are difficult to constrain in an actual landscape. This study presents a method to estimate adjustment timescales of width and hillslope angle and apply the method to rivers around the Yunodake fault, an active normal fault in Fukushima, northeastern Japan.
We focused on channel slope (ksn: Normalized channel steepness), channel width, hillslope angle and examined their variation along the trunk streams. Rivers flowing across the fault contained slope-break knickpoints. The lower channel reaches had larger ksn than reaches upstream of the knickpoints, suggesting these rivers are now adjusting to the accelerated relative uplift rates of the Yunodake fault. We measured bankfull width using a laser range finder. Although channel width increases with drainage area at a steady state, the observed channel width for three trunk streams did not follow such scaling relations. As for hillslope angle, we mapped hillslopes adjacent to the trunk streams and examined its along-trunk variation of hillslope angles using 5m-DEM provided from the Geospatial Information Authority of Japan. The result showed hillslopes downstream were steeper than those upstream, and there were transitional zones at the middle of the rivers where hillslope angles gradually increased toward the downstream side. We analyzed the 10Be concentrations of detrital sand taken from the trunk streams to determine basin-averaged denudation rates and then calculated knickpoint travel speed using the resulting denudation rates. Based on the observations of channel and hillslope morphology and the knickpoint travel speed, we will discuss the adjustment timescales of channel width and hillslope angles and their significance in terms of a basin-scale steady state.